Security enclosures may be used in the protection of items in transit or storage such as documents or magnetic information, and may be used to contain components to which it is desired to restrict access, such as electronic circuits containing memory information. Documents and magnetic files containing security sensitive information have to be sent throughout the world and it is often necessary to ensure that the documents have not been interfered with in transit or that if any attempt is made to reveal the information by an unauthorised person an alarm is sounded and the information destroyed or erased. Printed circuit boards, computers and data conversion equipment often contain memory devices which must be secured against unauthorised access. Any attempt to access or tap onto the board or device should ideally result in erasure of any memory information.
The enclosure should provide protection from many forms of attack, from very crude destructive attacks to highly sophisticated laboratory attacks, but at the same time be sufficiently insensitive as not to be triggered by normal environmental effects such as temperature, humidity and vibration. Such a security enclosure is described in U.K. patent application GB 2220513A to W. L. Gore & Associates Inc. The disclosed enclosure is formed from layers of flexible material. One layer carries flexible semi-conductive lines arranged to extend over the whole area of the enclosure. Any interruption of the lines by unauthorised opening of the enclosure changes the resistance of the line and so may be detected by a monitoring circuit. Also, two layers of semi-conductive fibers cover the whole area of the enclosure and are separated by an insulating layer. The length of the fibres forming the layers is of greater than the thickness of the insulating layer so that if the enclosure is pierced fibres from one layer will be forced into contact with fibres from the other layer. This will change the combined resistance of the layers and this may also be detected by the monitoring circuit. In the embodiment disclosed in this application, the layer carrying the flexible semi-conductive lines forms the second outermost layer of the enclosure, being covered by a further insulating layer of polyester.
One method of penetrating the disclosed enclosure would involve isolating selected flexible semi-conductive lines so that they could be cut through without the breaks in the lines being detected by the monitoring circuit. To gain the necessary access to the semi-conductive lines it would be necessary to peel back selected areas of the outer insulating layer.
A further form of security enclosure, in the form of a container, is disclosed in British Patent No. 1375926 to GAO Gesellschaft Fur Automation und Organisation mbH. The inside faces of the walls of the disclosed container are lined with an insulating foil on which conductors may be deposited by evaporation or by etching. The conductors are monitored by measuring the currents flowing in the conductors, a change in current indicating interference with the container. A plurality of foils may be glued on to one another and the adhesion of a conductor web on the foil is less than the adhesion which is produced when one foil is glued on to the other with a particular adhesive. As a result, when one foil is separated, the conductor web on the other foil may be positively torn away.